Film and sheet products are often stressed and cold formed during the assembly process in many applications. For example, in some appliance applications, opaque and colored films are often laminated to metal in place of paint. In this case, the polymer film is laminated to the metal in a flat form. The metal with the polymer film lamination is then bent or pressed into shape to form appliance housings for washers, dryers, refrigerators, and the like. During this cold forming process, some polymer film formulations tend to whiten as they are bent and stretched into shape and cause unattractive white streaks to appear in the finish of the appliance. This problem is more pronounced with darker colors. Stress whitening behavior is usually determined visually. A film is usually stretched to 50% or 100% elongation and examined for color changes. Flexible film and sheet products that do not exhibit stress whitening properties are therefore highly desirable for many applications.
Calendering is an economic and highly efficient method to produce film and sheet from plastics such as plasticized and rigid poly(vinyl chloride), abbreviated herein as “PVC”, and polypropylene compositions. The film and sheet usually have a thickness ranging from about 1 mil (0.025 mm) to about 80 mils (2.0 mm). Calendered PVC films or sheets are readily thermoformed into various shapes, which can be used in a wide range of applications including packaging, pool liners, graphic arts, transaction cards, security cards, veneers, wall coverings, book bindings, folders, floor tiles, and products that are printed, decorated, or laminated in a secondary operation. Additional discussion on polypropylene resin compositions used in calendering processes may be found in Japanese Patent Application No. Hei 9-40823 and European Patent Application No. 0 744 439 A1.
By contrast, conventional processing of polyesters into film or sheet involves extruding a polyester melt through a manifold of a flat die. Manual or automatic die lip adjustment is used to control thickness across a web of material. Water-cooled chill rolls are used to quench the molten web and impart a smooth surface finish.
PVC compositions are, by far, the largest segment of the calendered film and sheet business. Small amounts of other thermoplastic polymers such as, for example, thermoplastic rubbers, certain polyurethanes, talc-filled polypropylene, acrylonitrile/butadiene/styrene terpolymers (ABS resins), and chlorinated polyethylene, are sometimes processed by calendering methods. By contrast, polyester polymers such as, for example, poly(ethylene terephthalate), abbreviated herein as “PET”, or poly(1,4-butylene terephthalate), abbreviated herein as “PBT”, are difficult to calender successfully. For example, PET polymers with inherent viscosity values of about 0.6 deciliters/gram (abbreviated herein as “dL/g”), typically have insufficient melt strength to perform properly on the calendering rolls. Melt strength is defined as the ability of a polymer to support its weight in the molten state. In calendering, melt strength is related to the ability to remove the film from the roll process without deformation. For example, when calendered, a polymer with low melt strength will quickly sag and hit the floor; whereas, a polymer with high melt strength will maintain its shape for a much longer amount of time and can be further processed. Melt strength is thus important to minimize the amount of “drawdown” and gravity-induced sagging the polymer experiences during the calendering process. Drawdown is defined in calendering as the amount of thickness reduction between the calendering rolls and the take-up system and is expressed as the ratio of the nominal thickness or width dimension as the film exits the calendering rolls with the same dimension at the take-up roles.
PET and other polyester polymers also are prone to crystallize at typical processing temperatures of 160° C. to 180° C., resulting in a non-homogeneous mass which also causes high forces on the calender bearings. Increasing processing temperatures will reduce melt viscosity and improve processability. Higher temperatures, however, can cause degradation of the polyester such as, for example, by thermal degradation, hydrolysis of polymer by exposure to atmospheric moisture, and the formation of color bodies. Typical PET polymers also have a tendency to stick to the calendering rolls at higher processing temperatures. The calendering of various polyester compositions and several approaches to these problems has been described, for example, in U.S. Pat. Nos. 5,998,005; 6,068,910; 6,551,688; U.S. patent application Ser. No. 10/086,905; Japanese Patent Application Nos. 8-283547; 7-278418; 2002-53740; 10-363-908; 2002-121362; 2003-128894; 11-158358; and WO 02/28967. Although some of these difficulties can be avoided by the careful selection of polymer properties, additives, and processing conditions, calendering of polyesters at high rates of production is difficult.
The above shortcomings of the art illustrate the need for film or sheet products that may be produced by extrusion or by calendering and that do not exhibit stress whitening. These products would have applications as wood and metal laminates, graphic arts, transaction cards, security cards, veneers, wall coverings, book bindings, folders and the like.